Interior strapping, insulation installation
On my new Wisconsin home I plan to strap the 2×6 studs on the inside with horizontal 2x2s to break thermal bridges and facilitate the install of vertical tongue-and-groove paneling inside my home. I plan to use fiberglass batts.
I will probably go with an R-19 in the 2×6 cavity. But, what in the 2×2 cavity? Thin batts seem hard to find. Should I just use an R-11 batt and squeeze it into the cavity, compressing the R-19 batts a bit too? Will the higher density be good and increase the R-19 some too?
By the way, the house is sheathed with 1-inch blue foam on the outside, taped and Tyveked under HardiPlank siding. The attic will have blown cellulose.
Thanks,
john the toymaker.
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Replies
John,
If you've decided to blow cellulose in your attic, it seems that the answer to your question is fairly obvious: you should also blow cellulose in your wall cavity.
If you are installing T&G boards as your interior wall finish, don't forget to install an air barrier -- usually that means drywall -- under your boards.
Martin, Thanks for responding. I considered cellulose in the walls but was worried about settling in the 7 inch cavity with a poly air barrier on the inside and foam on the outside. Without some "rough" surfaces wouldn't its propensity to settle be more pronounced. Alternatively damp spray seems to leave too much moisture in the cavity and that concerns me as well. Do you think squeezing fiberglass batts a problem? Will that enhance or subtract from the R value? thanks john
John,
No matter what type of insulation you choose, you shouldn't install an interior polyethylene vapor barrier. Walls like yours with exterior foam sheathing need to be able to dry to the interior.
If you install dense-packed cellulose behind air-permeable netting (installed at 3.5 pounds per cubic foot), you won't get any settling.
Compressing fiberglass batts reduces the R-value of the batt but raises the R-value per inch.
Martin,
From what I can determine 1 inch non-faced xps foam is about 10 times a permeable as poly and in Winter in Wisconsin interior moisture seems far more likely to be moving into the wall than any outside-in movement from external sources, especially with a one story design with wide overhangs all around. I plan to be zealous in sealing the poly and plan an HRV. Under these circumstances can't the wall dry to the exterior and poly make sense? Again, thanks for your time. john
John,
You are bucking decades of research and flying in the opposite direction from the best building scientists in the world ... but hey, it's your house.
I think you are nuts to install interior poly.
For more information, delve into the huge treasure trove of technical papers at http://www.buildingscience.com.
Or, here at GBA, you might be interested in:
Vapor Retarders and Vapor Barriers
When Sunshine Drives Moisture Into Walls
Calculating the Minimum Thickness of Rigid Foam Sheathing
John,
Other problems with your wall:
1. You are in Climate Zone 6, so your foam sheathing needs a minimum of R-value of R-11.25 to prevent moisture accumulation in your wall assembly, so your 1 inch XPS is too thin. Thin exterior foam is risky.
2. One inch of XPS has a permeance of about 1 -- that's the old definition of a vapor retarder (formerly known as a vapor barrier) in all of the old code books. That foam sheathing isn't going to allow much drying to the exterior.